Send the site collection via rabbitmq, not via the database

This change solve neatly the performance issue encountered by Hannover RE (https://bugs.launchpad.net/oq-engine/+bug/1288262): instead of performing expensive geospatial queries N times, where N is the number of tasks, we perform the site model query only once in the controller node and then we send the site collection to the workers.

The tests pass: https://ci.openquake.org/job/zdevel_oq-engine/344/
A performance analysis on the cluster shows that the approach works well (actually better than before) even with half million sites or more.